Variable Contributions of Solar Energetic Protons, Helium, Heavy-Ions and Electrons to the Ionization of the Upper Atmosphere

During large Solar Energetic Particle (SEP) events such as that on April 21, 2002, particles precipitating into the upper atmosphere can have significant effects on atmospheric chemistry. Using data from ACE and SAMPEX the composition and energy spectra of SEPs from H to Fe with energies from ∼0.1 to ∼100 MeV/nuc and electrons from ∼0.04 to 14 MeV were measured during the April 2002 "Storms" period. Of these species, protons made the largest contribution to the ionization of the upper atmosphere at all energies, as is commonly assumed. However, electrons of similar penetrating power contributed ∼10% to ∼40% of protons, depending on energy, and He and heavier ions contributed ∼10% and ∼1-2% of protons, respectively. We have surveyed the relative contributions of these species in a number of other large events from 1997 to 2002 and find that the relative contributions of He and heavier ions are sometimes several times greater than in the 4/21/02 event, while electron contributions (relative to protons) can be up to ∼10 times greater. These results suggest that models of SEP energy deposition and associated chemical effects in the atmosphere may need to include other SEP species besides protons.